A discussion elsewhere brought this issue up, and these numbers should be posted for reference.

Replacing diesel with LNG requires roughly the energy equivalent of methane, plus whatever it takes to purify the gas and convert it to liquid. The info on liquefaction energy is hard to find; Linde Engineering doesn't even mention energy cost in its promotional material on its LNG plants. But I found a paper on Russian stuff which supplies a graph on page 15. This indicates about 250 Wh/kg at typical temperatures. This figure will increase for smaller, less-efficient systems, so figure 0.5 kWh/kg for a truck-stop sized unit. 1 kg of natural gas has 13.83 kWh of energy (47,200 BTU) so it takes about 2.9 kg of LNG to replace a gallon of diesel. This gas takes 1.4 kWh to liquefy. If this is done with electricity supplied from a CCGT powerplant at 50% efficiency delivered, it takes another 0.59 kg of gas per gallon-equivalent; if the power is generated on-site from e.g. a Capstone C60 gas turbine at 30% efficiency, it takes another 0.99 kg of gas per gallon-equivalent.

Replacing 70% of the 3037,000 bbl/day of distillate used for transport in 2007 (46.6 billion gpy @ 138,000 BTU/gal average for US distillate per EIA) would need 114 to 127 billion kg of natural gas, depending on the liquefaction overhead. This is 5.4 to 6.0 quads of gas. The USA produced ~21 trillion cubic feet in 2009; at 1020 BTU/scf, this is 21.4 quads of gas. Substituting for just 70% of diesel with LNG (no gasoline) would require increasing NG production by at least 25%. This may be possible, but it will require much higher NG prices (which are coming anyway).

Electrification needs less. If a dual-mode semi-truck averages 1.5 kWh/mile and traffic is 20% greater than the 2001 figure of 135.4 billion miles, annual electric power requirements would be 244 billion kWh, or about 6% of US demand for a complete replacement of diesel (not just 70%). Supplying this from NG using CCGT's at 50% efficiency delivered to the vehicle would require 3.53 billion kg of natural gas, or 1.67 quads. This is far more efficient, and the electric system can also use electricity from anything else on the grid. Finally, moving trucks to dual-mode rail eliminates pavement damage and cuts road-repair costs. The electric rail system is a better target for policy than converting semis to LNG.

Why would you want to use LNG as a vehicle fuel when it requires lots of refrigeration, is very cold, −162 °C (−260 °F), and requires careful handling. Why not turn it into Methanol, DME or some other higher order alcohol or ether, which might be a bit more tractable.?

GTL is about 45% efficient per Robert Rapier (he did a tour of a new GTL plant in Indonesia and that's what they got after working the bugs out; they started at about 33%). If LNG is too inefficient, we sure aren't going to make it with GTL.

While I agree with your position that electrified rail freight makes better sense I suspect that nevertheless, there will be a place for natural gas given that there are complications in replacing all truck based freight with rail (particularly to more remote areas not served by rail).

Your calculations are more or less in line with mine: I estimate about 6TCF of production will be required to replace about a million barrels which is more or less what the big rig fleet uses in North America.

It's interesting to note that at least one big-rig truck manufacturer already produces a natural gas version of it's vehicles:http://domesticfuel.com/2009/07/07/freightliner-trucks-introduces-first-natural-gas-powered-truck/

That said, we can release some of the oil sands crude to power our big rigs by substituting electric cars or mid-range trucks, which is easier to do.

I'm a little late to the party here, but I'd like to say that I'm very glad that you've posted this and your previous article on rail electrification. You may already be aware of my leanings toward dual-mode (or 'hi-rail' as I've called it) trucking from comments on Next Big Future.

Changing what freight trucks burn might buy the industry time (and with technologies like underground coalbed gasification, hydraulic fracturing, and so on, it might buy the industry a lot of time) but eventually, whether it's in ten years or fifty years, we're going to have to get serious about electrification. If we don't have huge, cheap batteries by then, overhead lines on the highway would be just the thing to keep the trucks moving. (And as you've pointed out, it would be a much better use of the gas even if the network isn't powered by a nuclear-renewables tag team at first.)

Yes, it did. Given today's cheap LNG, various air quality-related state incentives (California) and what I would suspect is a lack of highway tax on LNG, this makes a lot of sense for the company given today's policies and infrastructure. That does not make it the best option for the future.

"eventually, whether it's in ten years or fifty years, we're going to have to get serious about electrification. If we don't have huge, cheap batteries by then, overhead lines on the highway would be just the thing to keep the trucks moving."

It's not just the cost of petroleum, it's the cost of pavement. Maintaining concrete and asphalt which gets pounded by heavy trucks has depleted many highway trust funds and threatens to break budgets. Steel rail can handle multiples of semi-truck weights with far less wear; changing to dual-mode trucks and converting medians to railbed (or crushing damaged pavement for ballast) will be cheaper in the long run as well as more efficient due to lower rolling resistance. Electrification is the icing on the cake, removing petroleum from the equation.

Lower maintenance and lower energy costs are just two of a host of benefits a dual-mode system could have for both freight and passenger transportation. Aside from the money it would cost to build it there's almost no way I can think of that it isn't a huge win.

3. Segregated truck traffic doesn't have to compete with commuter traffic, again shortening trip times and improving highway safety. (But dual-mode vehicles can still use the normal highway if outages occur.)

4. Trucks on isolated rails could also travel at much higher speeds, yet again shortening trip times.

5. Guideways or rails imply largely automated operation. This should reduce driver fatigue and lead to safer operation at the last mile.

"Platooning" systems are under test today. If guidance is taken care of by the rails and only following distance needs to be controlled, drivers could sleep while the vehicle covers ground. Closely-spaced trains of vehicles would have lower air drag and carry more traffic on a single lane than several lanes of pavement.

"Buses could use it too."

Not just buses. Box trucks, motorhomes, heavy pickups... anything big enough to carry the rail wheels and power-handling gear. I suggested motorhomes in a post a while back.

Freeways aren't the only place this might work. Commercial vehicles could cruise oil-free and snag an opportunity charge from streetcar tracks in urban areas. If a delivery truck is always charging en route, it needs a much smaller electric range (and battery). Cost down, service time up, payload up.